Typical reason of Boiler tubes failure
1. Caustic Attack
Symptoms: Localized wall loss on the inside diameter (ID) surface of the tube, resulting in increased stress and strain in the tube wall.
Causes: Caustic attack occurs when there is excessive deposition on ID tube surfaces. This leads to diminished cooling water flow in contact with the tube, which in turn causes local under-deposit boiling and concentration of boiler water chemicals. If combined with boiler water chemistry upsets of high pH, it results in a caustic condition which corrosively attacks and breaks down protective magnetite.
2. Oxygen Pitting
Symptoms: Aggressive localized corrosion and loss of tube wall, most prevalent near economizer feedwater inlet on operating boilers. Flooded or non-drainable surfaces are most susceptible during outage periods.
Causes: Oxygen pitting occurs with the presence of excessive oxygen in boiler water. It can occur during operation as a result of in-leakage of air at pumps, or failure in operation of preboiler water treatment equipment. This also may occur during extended out-of-service periods, such as outages and storage, if proper procedures are not followed in lay-up. Non-drainable locations of boiler circuits, such as superheater loops, sagging horizontal superheater and reheater tubes, and supply lines, are especially susceptible.More generalized oxidation of tubes during idle periods is sometimes referred to as out-of-service corrosion.Wetted surfaces are subject to oxidation as the water reacts with the iron to form iron oxide. When corrosive ash is present, moisture on tube surfaces from condensation or water washing can react with elements in the ash to form acids that lead to a much more aggressive attack on metal surfaces.
3. Short-term Overheat
Symptoms: Failure results in a ductile rupture of the tube metal and is normally characterized by the classic “fish mouth” opening in the tube where the fracture surface is a thin edge.
Causes: Short-term overheat failures are most common during boiler start up. Failures result when the tube metal temperature is extremely elevated from a lack of cooling steam or water flow. A typical example is when super-heater tubes have not cleared of condensation during boiler start-up, obstructing steam flow. Tube metal temperatures reach combustion gas temperatures of 1600°F or greater which lead to tube failure.
4. Long-term Overheat
Symptoms: The failed tube has minimal swelling and a longitudinal split that is narrow when compared to short-term overheat. Tube metal often has heavy external scale build-up and secondary cracking.
Causes: Long-term overheat occurs over a period of months or years. Superheater and reheat superheater tubes commonly fail after many years of service, as a result of creep. During normal operation, alloy superheater tubes will experience increasing temperature and strain over the life of the tube until the creep life is expended. Furnace water wall tubes also can fail from long-term overheat. In the case of water wall tubes, the tube temperature increases abnormally, most commonly from waterside problems such as deposits, scale or restricted flow. In the case of either superheater or water wall tubes, eventual failure is by creep rupture.
Symptoms: Tube experiences metal loss from the OD of the tube. Damage will be oriented on the impact side of the tube. Ultimate failure results from rupture due to increasing strain as tube material erodes away.
Causes: Erosion of tube surfaces occurs from impingement on the external surfaces. The erosion medium can be any abrasive in the combustion gas flow stream, but most commonly is associated with impingement of fly ash or soot blowing steam. In cases where soot blower steam is the primary cause, the erosion may be accompanied by thermal fatigue.
6. Stress Corrosion Cracking (SCC)
Symptoms: Failures from SCC are characterized by a thick wall, brittle-type crack. May be found at locations of higher external stresses, such as near attachments.
Causes: SCC most commonly is associated with austenitic (stainless steel) super heater materials and can lead to either trans granular or inter granular crack propagation in the tube wall. It occurs where a combination of high tensile stresses and a corrosive fluid are present. The damage results from cracks
that propagate from the ID. The source of corrosive fluid may be carryover into the super heater from the steam drum or from contamination during boiler acid cleaning if the super heater is not properly protected.
7. Waterside Corrosion Fatigue
Symptoms: ID initiated, wide trans granular cracks which typically occur adjacent to external attachments.
Causes: Tube damage occurs due to the combination of thermal fatigue and corrosion. Corrosion fatigue is influenced by boiler design, water chemistry, boiler water oxygen content and boiler operation. A combination of these effects leads to the breakdown of the protective magnetite on the ID surface of the boiler tube. The loss of this protective scale exposes tube to corrosion. The locations of attachments and external weldments, such as buckstay attachments, seal plates and scallop bars, are most susceptible. The problem is most likely to progress during boiler start-up cycles.
8. Fireside Corrosion
Symptoms: External tube metal loss (wastage) leading to thinning and increasing tube strain.
Causes: Corrosion occurs on external surfaces of water wall tubes when the combustion process produces a reducing atmosphere (substoichiometric).This is common in the lower furnace of process recovery boilers in the pulp and paper industry. For conventional fossil fuel boilers, corrosion in the burner zone usually is associated with coal firing. Boilers having maladjusted burners or operating with staged air zones to control combustion can be more susceptible to larger local regions possessing a reducing atmosphere, resulting in increased corrosion rates.
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